This invention relates to a bidirectional data communication system which transmits and receives data bidirectionally between a central unit and a number of terminal units.
In the past, bidirectional data communications systems having many terminals communicating with a head-end have employed data buffers at each location and have usually separately generated clock frequencies for each direction of transmission. The buffers and clocks are needed to compensate for differences in data rates that may be used in the data processing circuits, and to compensate for arbitrary phase delays that may occur in transmission. In CATV systems, for example, the transmission lines and switching systems between the head-end and individual terminals tend to introduce wide variations in the phase of signals from different points in the system. Thus various compensating techniques have been proposed, including upstream transmissions using a central clock. Such systems however require extensive circuitry, as do systems employing individual clocks or buffers. Furthermore, when a great many subscriber terminals are coupled into the system, there should not be substantial delays in the interrogation sequence, as this would limit the number of subscribers that can be coupled into given head end.
In view of such disadvantages of the conventional data communication system, one object of the present invention is to provide a bidirectional data communication system which does not need clock signal generating circuits and buffer circuits in the respective terminal units and also which is less expensive in practical use as an entire system.
Another object of the present invention is to provide a bidirectional data communication system which can accurately read data transmitted with substantial phase variations to a head end unit from respective terminal units.